The invention relates to cellular telephone systems and particularly to intermediate frequency (IF) circuits utilizing a laterally-coupled double-mode resonator surface acoustic wave (SAW) filter to perform the function of received signal channel selection.
IF circuits of various kinds are well-known in radio receiver technology. They generate a difference frequency, called an intermediate frequency, or IF frequency, which actually is a "difference frequency" between a modulated rf signal received via an antenna and a "local frequency" generated by a so-called "local oscillator". The generation of the IF frequency signal eliminates the rf carrier of the modulated rf signal, and the IF frequency signal contains all of the modulation information representing information which is to be decoded to produce audio frequency signals. Most prior IF circuits use monolithic crystal filters to perform the function of intermediate frequency channel selection.
A few prior IF circuits used for television receivers utilize SAW devices as filters. Although SAW filters have been used in IF circuits for television receivers, television receivers have relatively large input signal levels and high signal-to-noise ratios. Television receivers also have wide bandwidths and channel separations, of the order of one megahertz or greater. Furthermore, television receivers do not share the same antenna with a transmitter. All of these factors make it practical to use the prior wide bandwidth, high insertion loss SAW filters in television receives.
However, SAW filters ordinarily would not be considered to be suitable for use in a cellular telephone transceiver because present regulations allow only 30 kilohertz separation between adjacent bands in a cellular telephone system, and because the cellular telephone receiver must function properly for signal levels in the-116 dbm to 0 dbm level, and must share an antenna with a transmitter capable of producing up to three watts of output power. Prior SAW filters generally produce so much insertion loss that use of such a filter would result in the necessity of re-amplifying the signal, which inherently would unacceptably increase the amount of noise in the signal and reduce the already low signal-to-noise ratio.
Thus, despite extensive development of circuit design technology in the fields of IF circuits, bandpass filter circuitry, and SAW filters, there still is considerable room for improvements that can allow use of more effective, lower cost components in receiver circuits, especially ones for use in narrow band, low level signal applications such as cellular telephone transceivers.
The closest known prior art utilizes SAW filters in television IF applications, in which high loss, larger bandwidth SAW devices are used for filtering. Another known type of SAW filter is referred to as a "double mode" resonator. This type of SAW filter traditionally has been used in oscillators. Double mode SAW resonators result in much lower insertion loss than traditional SAW filters, and also have much narrower bandwidth. Their circuit "Q" approaches 15,000, 150% greater than conventional SAW filter design technologies.
The bandwidth of SAW filters which have been used in prior IF circuits of television receivers typically is approximately one megahertz. The EIA (Electronics Industry of America) cellular telephone standard IS19B has established specifications for adjacent channels and alternate channels in cellular telephone systems, wherein adjacent channel center frequencies are separated by 30 kilohertz, and alternate channel center frequencies are separated by 60 kilohertz, much narrower than the bandwidth of SAW filters in prior IF circuits of television receivers.
In contrast, in the television receiver environment, the transmitters produce signals of thousands of watts, with ranges of hundreds of miles. This environment presents far different constraints on the design of receiver circuitry than is the case for the design of receivers for the cellular telephones in which the receivers are included in the same circuit with transmitters. Cellular telephone base stations typically transmit signals of only 15 watts, with ranges of 10 to 15 miles. The cellular television receiver is incorporated in a transceiver, the transmitter of which transmits up to three watts on the same antenna on which the incoming signals with levels as low as-116 dbm are being received, so if a SAW filter were to be used in a cellular telephone transceiver, the SAW filter would have to also alternate the three watt signals being transmitted on the same antenna by the transmitter section of the transceiver as well as provide proper selectivity, without substantial insertion loss, between 30 kilohertz channels. The maximum power received by cellular telephone transceivers is typically-30 dBm (maximum), which is a very low power level for a receiver section of a transmitter that is capable of producing 34.77 dBm (three watts). The above EIA cellular telephone specification requires a SINAD of 12 db for a-116 dBm input signal with 1 kilohertz modulating frequency having a deviation of.+-.8 kilohertz for cellular telephone transceivers. (Note that: ##EQU1##
Until very recently, there were no available double mode SAW resonators capable of operating with sufficiently low insertion loss to be considered for utilization in an FM receiver designed for receiving signal levels as low as would be required for a cellular telephone system.
The state of the art for SAW filters is set forth in "Surface Acoustic Waves for Signal Processing" by Feldmann and Henaff, published by Artech House, Boston and London, 1986, English translation 1989. A double mode resonator, shown in FIG. 4 hereof, consists of two interdigital transducers (IDT's) 48 and 51 both located in a resonant cavity. The electric input signal is transformed into a surface acoustic wave by the IDT 48. The acoustic wave propagates through the cavity along reflector pattern 49 and reflector pattern 50 and is detected by the second IDT 51. The IDT spacings, the cavity size and other factors interplay to produce the insertion loss. From a "black box" viewpoint, a dual mode SAW resonator functions as a dual port filter having lower insertion loss and much narrower bandwidth than previously available in any type of prior SAW filter.